Apparatus, and process for cold spray deposition of thermoelectric  semiconductor and other polycrystalline materials and method for making polycrystalline materials for cold spray deposition

ABSTRACT

An apparatus and method perform supersonic cold-spraying to deposit N and P-type thermoelectric semiconductor, and other polycrystalline materials on other materials of varying complex shapes. The process developed has been demonstrated for bismuth and antimony telluride formulations as well as Tetrahedrite type copper sulfosalt materials. Both thick and thin layer thermoelectric semiconductor material is deposited over small or large areas to flat and highly complex shaped surfaces and will therefore help create a far greater application set for thermoelectric generator (TEG) systems. This process when combined with other manufacturing processes allows the total additive manufacturing of complete thermoelectric generator based waste heat recovery systems. The processes also directly apply to both thermoelectric cooler (TEC) systems, thermopile devices, and other polycrystalline functional material applications.

This present application is a Continuation of U.S. application Ser. No.16/399,560 filed 30-Apr.-2019, the entire contents of which beingincorporated by reference herein in its entirety, and also claims thebenefit of the earlier filing date of U.S. Provisional Application No.62/673,556 filed 18 May 2018, the entire contents of which beingincorporated by reference herein in its entirety. The presentapplication also contains subject matter related to that in U.S. Pat.No. 9,306,146, the entire contents of which is incorporated herein byreference.

BACKGROUND

Waste heat is a significant global environmental issue, but it is onethat draws little attention because of very limited solutions. A largefraction of energy used in industrial processes and transportationsystems is lost as low grade waste heat, and several trillion dollars ofenergy generated from fossil fuels in the U.S. each year, equal tonearly sixty quadrillion BTUs, is discarded without benefit in the formof waste heat.

Thermoelectric generators or (TEGs) using solid state conversion of heatto electricity is one technology for application to waste heat recovery,but TEGs are currently only used in niche markets because the resultingcost per kilowatt is high, and they are difficult to integrate intowaste heat sources because of their flat plate form factor. TEGs areusually manufactured in small, flat plate modules using small pellets ofN-type and P-type crystalline semiconductor materials wired in series orparallel. As recognized by the present inventor, the flat form factormakes the TEGs difficult to thermally couple to complex shaped heatsources and equally difficult to couple to active cooling sources, bothissues adding significantly to TEG installation complexity and cost. Thepresent inventor also recognized that TEGs made using semi-conductorpellets also require the use of solders to hold the multilayerthermoelectric couple element assembly together, and these materialsoften limit potential applications and the maximum electrical output ofthe devices. The overall design of current commercial thermoelectricdevices also make them subject to degradation or failure when subjectedto moderate to intense mechanical and thermal shock environments.

Additive manufacturing of TEGs offers the potential to spray orotherwise deposit the N-type and P-type semiconductor materials and allthe other material layers required for a functional TEG directly ontocomplex shaped waste heat or other thermal energy sources. The potentialcombination of several additive manufacturing processes enablesequentially building up the electrical isolation layer, an adhesionlayer, the interconnecting conductive metallic layer or layers,diffusion barrier material layer, and both the N-type and P-typesemiconductor layers required for a functional TEG. One potential methodof achieving both the deposition of the thermoelectric semiconductor andmetallic materials is the use of the supersonic cold-spray depositionprocess, although the industry has struggled to find a solution thatachieves that objective.

“Supersonic cold spray” is a material deposition process that has beendeveloped to build up metallic material layers by impacting micrometersized metal particles at high velocities onto a substrate. A helium ornitrogen gas stream under pressure is accelerated to supersonic velocityby expansion through a converging-diverging nozzle. The normallyspherical metal particles of the material being deposited are insertedinto the gas stream either in the converging or diverging sections ofthe nozzle and then accelerated to high velocity. The normally sphericalmetal particles in the size range from 10-80 micrometers becomeentrained within the gas and are directed towards the surface where theydeform and knit together on impact forming a strong bond with thesurface and with each other. Gas type, gas pressure, gas temperature,nozzle configuration, nozzle extension length, average particle size ofthe material being sprayed, the particle's drag coefficient, and theparticle size distribution must be optimized for each differentmaterial. In addition, the feed mechanism and the feed rate of thepowdered material into the gas stream must be tailored to the materialbeing sprayed. A unique advantage of the cold-spray process is that theparticles are maintained below their melt temperature, and successfuldeposition depends on the micrometer sized, normally spherical, metalparticles deforming on impact. Thus, implementation of the cold sprayprocess has been primarily focused on the use of metallic materials,materials that are malleable and that can be hammered or pressedpermanently out of shape without breaking or cracking and particles thatcan be fused or forged together below their melt temperature. For thatreason, the cold-spray process has not been generally applied to thedeposition of crystalline or polycrystalline materials, andthermoelectric semiconductors materials and other energy harvestingsemiconductor materials. In addition, in the deposition of metals, themetal particle sizes are generally restricted to being greater than tenmicrons in diameter and normally in the range between 25 microns and 75microns in diameter since they must exhibit sufficient drag area andmass to be accelerated by the gas stream and gain sufficient momentum tohit the surface with enough force to deform and adhere to the surfaceand each other before being swept away by the gas stream.

SUMMARY OF THE DISCLOSURE

Using a supersonic cold-spray process, the adhesion to the surface anddeposition of near theoretical density layers of both N-type and P-typethermoelectric semiconductor materials and other polycrystallineparticles has been achieved by controlling the shapes of the particlesin the thermoelectric semiconductor powder material, the maximumparticle size and the particle size distribution, incorporating a powderflow and deposition enhancing additive materials, utilizing specificallyconfigured cold spray nozzle and preheated pressurized gas process, andcontrolling the thermoelectric semiconductor peak particle velocities.Thermoelectric semiconductor powder materials, equipment design, andprocess parameters have been developed to enable and enhance thecold-spray deposition of thermoelectric semiconductor and othercrystalline materials onto metals, glass, ceramic, and high temperaturecapable polymer surfaces. The powder used in the supersonic cold sprayprocess is controlled to volumetrically have greater than 95% of theparticles with a maximum particle size not greater than 15 micrometersand a controlled volumetric particle size distribution over the nominalparticle size ranges, from 0.1-6.0 micrometers; 2.0-10.0 micrometers;and 5.0-15 micrometers in major dimension, and by the incorporation ofpowder flow and deposition enhancing additive materials, such as, butnot limited to, hollow glass microspheres, when combined with 1.0-2.0 mmdiameter cold spray nozzle throat dimensions, a nozzle throat area toexit area expansion ratios of 6-10, a nozzle divergent/convergent lengthratios between 1 and 3, a nitrogen gas process parameters of 450-550degrees centigrade pre-heat, and gas pressures from 0.4-0.9 millionpascal (MPa) yielding gas and particle velocities from 750 to 900meters/second, an adhesion to the surface and deposition of neartheoretical density layers of both N-type and P-type thermoelectricsemiconductor materials and other poly crystalline particles has beenachieved. Using this process, the semiconductor materials are depositedonto flat as well as complex shaped surfaces in both thin and thicklayers, and from individual small pellet sized spots to large continuousareas, thus enabling TEG designs specifically tailored to theapplication and the heat source. Supersonic cold-spray deposition ofthermoelectric materials such as bismuth telluride, antimony telluride,and Tetrahedrite type copper sulfosalts have been demonstrated, andthese processes can also apply to functional materials used in othersolid state energy harvesting techniques, such as but not limited topiezoelectric devices, thermopiles, as well as for thermoelectriccooling (TECs) devices. Unique powder material composition, particleshape and sizes, and control of the cold-spray process parameters andequipment design allow the uniform cold spray deposition to a surface ofpolycrystalline materials such as thermoelectric semiconductormaterials, and the developed process further enables and enhances thecold spray deposition of crystalline and other non-malleable particlesof irregular shapes and dimensions of less than 10 micrometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of one apparatus used for cold spray deposition ofmetallic materials.

FIG. 2A is a diagram that shows a shape and size of a typical metalsupersonic cold-sprayed particle just prior to impacting a metalsubstrate. FIG. 2B is a diagram that shows deformation that occurs inboth the metal particle and the metal substrate shown in FIG. 2A afterthe particle has impacted and then adhered to the substrate.

FIG. 3 is a diagram that shows a comparison of the material particleshape and sizes between metal particles which are currently used in theestablished supersonic cold spray process for metals, and the complexand irregular thermoelectric semiconductor particle shapes and sizesthat are created during a mechanical size reduction process ofpolycrystalline materials such as bismuth and antimony telluride andTetrahedrite, copper sulfosalt formulations.

FIG. 4A is a diagram that shows an exemplary range of hollow glassmicrosphere particles, which can be used as a modifier to facilitate thetransport of micrometer and nanometer sized and irregular shapedthermoelectric semiconductor particles into the supersonic gas streamand to the surface of the substrate as described in the currentdisclosure.

FIG. 4B is a diagram of an example thermoelectric semiconductor powdermix adhering to the hollow glass or other composition microspheres ofFIG. 4A to facilitate the cold-spray deposition of these very small,micron to nanometer sized polycrystalline thermoelectric semiconductormaterials as described in the current disclosure.

FIG. 5 is a flowchart of a process used to produce a thermoelectricsemiconductor powder that can be successfully deposited using thesupersonic cold spray process as described in the current disclosure.

FIG. 6 is a flowchart of elements of an overall supersonic cold sprayprocess and supersonic cold-spray system design parameters developed forthermoelectric semiconductor and other crystalline materials asdescribed in the current disclosure.

FIG. 7 is a diagram of a supersonic cold spray nozzle design that can besuccessfully used for the cold-spray deposition of thermoelectricsemiconductors and other crystalline materials as described in thecurrent disclosure.

FIG. 8A is a graph of a volumetric particle size distribution of P-typebismuth telluride particles that can be successfully cold-sprayed usingthe processes and equipment design parameters defined within thisdisclosure.

FIG. 8B is a table of cumulative volume % less than indicated size forboth P-type and N-type bismuth telluride thermoelectric semiconductorpowders that can be used for successful deposition using the cold-sprayprocesses and equipment design parameters defined within thisdisclosure.

FIG. 8C is a table of the cumulative number % less than indicated sizefor both P-type and N-type bismuth telluride thermoelectricsemiconductor powders that can be used for successful deposition usingthe cold-spray processes and equipment design parameters defined withinthis disclosure.

The images of FIGS. 9A and 9B, and tables in FIGS. 9C and 9Dcumulatively demonstrate the effects of particle size and size rangevariables of thermoelectric semiconductor powders on the supersonic coldspray deposition process. Successful cold spray deposition ofthermoelectric semiconductor materials is highly dependent on limitingthe maximum particle size and controlling the particle size distributionof the powder material used in the process. Moreover, FIG. 9A is animage of a successful cold spray deposition of P-type bismuth tellurideon a glass substrate when using thermoelectric semiconductor powdermaterial with the particle size distribution shown in FIG. 9C andmanufactured as described in the current disclosure. FIG. 9B is an imageof a cold spray deposition of bismuth telluride on a glass surface whenusing a particle size distribution shown in FIG. 9D that does notconform to the current disclosure. FIG. 9C is a table of cumulativevolume % less than indicated size for both P-type and N-type bismuthtelluride thermoelectric semiconductor powders that can be used forsuccessful deposition using the cold-spray processes and equipmentdesign parameters defined within this disclosure. FIG. 9D is a table ofcumulative volume % less than indicated size for both P-type and N-typebismuth telluride thermoelectric semiconductor powders that cannot beused successfully in the cold-spray process, as this powder was notprepared within the full constraints of this disclosure.

FIGS. 10A, 10B and 10C are graphs generated at Lawrence LivermoreNational Laboratory that demonstrate that cold sprayed thermoelectricsemiconductor material prepared per the processes as defined in thisdisclosure retain their thermoelectric properties. FIG. 10A is a graphof Seebeck coefficient versus temperature for a P-type cold-sprayedbismuth telluride material created by the processes defined within thisdisclosure compared to a sample taken from the precursor P-typepolycrystalline billet that was used in the generation of the bismuthtelluride cold spray powder per the process shown in FIG. 5 as describedin the current disclosure. FIG. 10B is a graph of the Seebeckcoefficient versus temperature measured at Lawrence Livermore NationalLaboratory for an N-type cold sprayed bismuth telluride material createdby the processes described in this disclosure compared to the precursorpolycrystalline billet used in the generation of the bismuth telluridecold spray powder per the process shown in FIG. 5 as described in thecurrent disclosure. FIG. 10C is a graph of the thermal conductivity of aP-type cold sprayed bismuth telluride material created by the processesdescribed within this disclosure compared to the precursorpolycrystalline billet used in the generation of the bismuth telluridecold spray powder per the process shown in FIG. 5 as described in thecurrent disclosure.

FIG. 11A is an image of a chemically etched glass slide on which asupersonic cold-sprayed line of P-type bismuth telluride has beendeposited using the processes and equipment as described in the currentdisclosure. FIG. 11B is an image of a small section of 6061-T3 sheetaluminum on which a supersonic cold-sprayed line of P-type bismuthtelluride material has been deposited using the processes and equipmentas described in the current disclosure. FIG. 11C is an image of a smallsection of 110-ETP alloy copper plate on which a supersonic cold-sprayedline of P-type bismuth telluride material has been deposited using theprocesses and equipment as described in the current disclosure. FIG. 11Dis an image of a small section of sheet stainless steel on which asupersonic cold-sprayed line of P-type bismuth telluride material hasbeen deposited using the processes and equipment as described in thecurrent disclosure. FIG. 11E is an image of a small section of alow-density syntactic high temperature silicone resin foam on which asupersonic cold-sprayed line of P-type bismuth telluride material hasbeen deposited using the processes and equipment as described in thecurrent disclosure. FIG. 11F is an image of a small section of analuminum silicate ceramic material on which a supersonic cold-sprayedline of P-type bismuth telluride material has been deposited using theprocesses and equipment as described in the current disclosure.

FIG. 12 is an image of a circular cross-section of a copper pipe with anN-type bismuth telluride cold-sprayed element and a P-type cold-sprayedbismuth telluride element demonstrating the ability to cold-spraydeposit both N-type and P-type thermoelectric semiconductors on complexshaped surfaces using the processes and equipment as described in thecurrent disclosure.

FIG. 13 is a diagram of an apparatus according to another embodiment ofthe present disclosure using high performance magnets to focus the coldsprayed thermoelectric semiconductor particles and improve thedeposition.

DETAILED DESCRIPTION

Supersonic cold spray process can be used extensively for the depositionof metallic materials including aluminum, copper, nickel, and manyothers metals and their alloys. The process can be used for a variety ofapplications including metals repair, corrosion control, and theapplication of hardened surface coatings. Optimization of the finaldeposited material is achieved by the selection of numerous powdermaterial, equipment, and supersonic cold-spray process parameters.

FIG. 1 is a high-level diagram of one type of equipment currently usedfor the cold spray deposition of metallic materials. A gas 1 such as,air, nitrogen or helium is pressurized in the range of from 0.5-3.5 MPa.This pressurized gas 1 is directed into a heater 2 and is heated to300-550 degrees centigrade. This gas is then directed through aconverging-diverging nozzle 3 with a throat diameter between 2.0millimeter and 2.5 millimeter where the gas 1 is accelerated tosupersonic velocity. In this existing version of a cold spray system, alow-pressure gas 4 normally air at atmospheric pressure is fed into apowder feeder 5. The powder feeder 5 contains nearly spherical metalparticles 10 in a particle size range from 10-80 micrometers indiameter. The low-pressure gas 4 combines with the nearly sphericalparticles 10 and carries them through a powder entrance tube 6 insertedinto the diverging section of the converging-diverging nozzle 3 where itmixes into the gas 1 that has been accelerated to supersonic velocity.The nearly spherical metal particles 10 accelerated within thesupersonic gas flow are directed via 15 a nozzle extension toward asubstrate 7 where they are deposited as a coating 20 on impact. Specificnozzle throat area to exit area, convergent-divergent ratios, powderentrance tube diameters and entrance angles are proprietary aspects ofeach cold spray equipment vendor's design. Despite the variation innozzle convergent/divergent ratios, as per vendor designs, the presentteachings of this disclosure are applicable to the variations. Forexample, a smaller nozzle throat diameter aids in reducing the gasvolumetric flow rate through the nozzle orifice, which is beneficialwhen spraying smaller sized particles, and an increasing divergent toconvergent length ratio tends to increase the peak particle velocity atspecific input gas pressure and temperature conditions, which furtheraids in the transport of smaller particles to the surface.

FIG. 2A shows the shape and size of a typical metal particle 200 thathas been accelerated to supersonic velocity by the cold spray systemshown in FIG. 1 just prior to impacting a metal substrate 201 surface.The typical metal particle 200, is in the size range of 10 to 80micrometers in diameter and normally constrained to a range from 25-75micrometers in diameter.

FIG. 2B shows the deformation that occurs to both the typical metalparticle 200 and the metal substrate 201 shown in FIG. 2A after thetypical metal particle 200 has impacted and adhered to the metalsubstrate 201. The resulting deformed particle 202 embeds within thedeformed substrate 203 and forms a strong bond with the substratematerial. The adhesion of the metal particles to the substrate as wellas the continued adhesion and buildup of new metal particles to thepreviously deposited material is accomplished in the solid state, andthe successful initial and continued deposition depends on themalleability of the metal particles.

The attributes of the supersonic cold spray process for metals includethe low temperature deposition, formation of a dense material structure,and material thermal, structural and electrical properties near or equalto the wrought or cast material. Metal powders with the addition ofsolid ceramic additives are optionally used to improve the hardness andwear resistance of the final materials where the deforming metalparticles encapsulate and hold the non-deforming ceramic additiveswithin the deposited material matrix.

The supersonic cold-spray deposition process used for metals does notdirectly apply to the cold spray deposition of thermoelectricsemiconductors, and more generally to crystalline materials. Crystallinematerials are not malleable, and they do not deform when they hit thesurface at high velocity. Micron sized particles made from crystallinematerials are not usually spherical in shape and uniform in size in allthree dimensions.

FIG. 3 is a comparison of the material particle shape and sizes betweenmetal particles which are used in supersonic cold spray process formetals, and the complex and irregular thermoelectric semiconductorparticle shapes and sizes that are created during a mechanical sizereduction process of polycrystalline materials such as bismuth andantimony telluride and Tetrahedrite, copper sulfosalt formulations.

In the supersonic cold-spray deposition of metals, a typical sphericallyshaped metal particle 300 in the diameter range from 25-75 microns insize is used. Particles 301 of crystalline materials in the 25-75micrometer size range can be made using mechanical sizing processes, butcrystalline particles of this maximum size, vary widely in shape andsize in all three dimensions. During the size reduction process, thebrittle nature of the crystalline material will also produce shard-like,micron sized particles 302 whose sizes vary between 1-6 micrometers inall three dimensions. The size reduction process can also produceextremely small particles 303 whose major and minor dimensions arenanometer in size. Cold spray testing, forming one of the bases for thisdisclosure, have shown that thermoelectric semiconductor particles 301,of shapes and/or dimensions used in the cold spray deposition of metalsdo not deform and do not adhere when they impact a surface at supersonicvelocity. Crystalline-semiconductor material particles in the 25-75micrometer size range instead can fracture and bounce off when theyimpact the surface, essentially sandblasting the surface.Crystalline-semiconductor particles of the size range from 25-75micrometers when cold sprayed will create craters and fractures in anysemiconductor material that has previously deposited as shown in FIG.9B. Subsequent impacts from particles of this size range eventuallyscrub away any buildup of semiconductor material that may havepreviously occurred.

The performance of thermoelectric semiconductor materials, and manyother energy harvesting semiconductors, critically depends onmaintaining both the crystalline atomic structure and a preciseelemental composition including dopant concentration necessary toproduce both N-type and P-type materials. Both the atomic structure andthe composition characteristics can be negatively compromised if thematerial is subjected to temperatures or other conditions sufficient tomelt the material, change its phase, or alter its composition. Forbismuth telluride and antimony telluride formulations, subjecting thematerials to temperatures above 580 degrees centigrade, and forTetrahedrite type materials subjecting the material to temperaturesabove 650 degrees centigrade can change the elemental composition andcrystalline structures sufficient to reduce the Seebeck coefficient,and/or increase the thermal conductivity and the resistivity of thematerial. If high temperature thermal spray techniques such as plasmaspraying, flame spraying, and high velocity oxy-fuel spraying (HVOF) areused for thermoelectric semiconductor materials, the particles melt andthose changes to the crystalline structure and/or the elementalcomposition can significantly reduce or even eliminate the desirablethermoelectric properties. Although supersonic cold-spray technologyoffers the potential attribute of low temperature deposition, pastattempts to directly deposit more than micrometer thick thermoelectricsemiconductor, and/or other crystalline powders using the cold spraytechnique have generally proved unsuccessful, and successful processesfor the reliable buildup of uniform, thick layers of material requiredfor applications such as thermoelectric generators has not beenachieved.

Particles made from both N-type and P-type polycrystallinesemiconductors have many characteristics that have hindered or preventedtheir effective use in the supersonic cold-spray process. 1) They areusually brittle, and they tend to fracture and disperse rather thandeform and adhere when impacting a surface or each other at supersonicvelocity. 2) Polycrystalline semiconductor particles of the size rangeused for metal cold-spray deposition, when they impact the surface, cancrater and sand blast away any previously deposited material. 3)Particles made by grinding or milling large polycrystalline billets ofmaterial are generally not spherical or regular in shape, withsignificant variations in the size of all three dimensions and shapeirregularities in each dimension. 4) The irregular, shard-like shapes ofthe crystalline particles have widely varying drag coefficientsdepending on their orientation within the supersonic gas stream so theirrapid and uniform acceleration to supersonic velocities in the gasstream is uncertain. 5) Very low mass particles in the nanometer to lowmicrometer size range may not gain sufficient kinetic energy to traversethe bow shock of the expanding gas stream and reach the surface. 6) Thecohesive and adhesive nature, electrostatic charge, hygroscopicity andnon-Newtonian flow characteristics of powders consisting of small,non-spherical, crystalline semiconductor particles, severely inhibit theuniform flow of the powder into the gas stream. 7) The manufacture ofpowders of thermoelectric semiconductor materials with a maximumparticle size and a controlled size range distribution is difficult. 8)The grain size and the orientation of the crystalline structure in thefinal deposited material are important in determining the finalthermoelectric properties.

Consequently, as recognized by the present inventor, the differences ineach of these material attributes, and their complex interactions whenused in the supersonic cold-spray deposition process, have shown thatthe successful deposition of thermoelectric semiconductor materialsrequires a revision to the understanding of the mechanism by which theseparticles adhere to the surface and to each other. That difference,recognized by the present inventor, drives the need for the significantreduction in the allowable particle sizes and size ranges within thesemiconductor material powders down to the very low micrometer to highnanometer size range, and the deposition process requires irregularityin the individual particle shapes. The combination of particle shape,and particle sizes developed for supersonic cold-spray of metallicmaterials, will not work for thermoelectric semiconductor and otherpolycrystalline materials. The current disclosure provides methods,processes and equipment design parameters that enable the cold-spraydeposition of thermoelectric semiconductor and other polycrystallinematerials to various metallic and non-metallic substrate materials.These disclosed methods and processes when combined with existingmanufacturing processes enable the additive manufacturing of completethermoelectric generator systems that can be used to recover energy fromcomplex shaped waste heat sources used in every economic sector. Themethods and processes also enable a significant number of applicationsfor providing power to Internet of Things (IOT) devices.

One of the difficulties associated with supersonic cold sprayingthermoelectric semiconductor particles is the high critical angle ofrepose of the powder and the resulting flow resistance of the irregularshaped particles. The resulting highly cohesive and adhesive nature of0.10-15 micrometer, highly irregular shaped semiconductor particlescreate a significant resistance to the uniform flow of the material intothe supersonic cold-spray nozzle, and flow enhancement additives suchas, but not limited to, hollow glass microspheres can be used to reducethe crystalline powder's resistance to flow.

FIG. 4A shows an exemplary range of hollow glass microsphere particles,which can be used as modifiers to facilitate the transport of micrometerand nanometer sized and irregular shaped thermoelectric semiconductorparticles into the supersonic gas stream and then to the depositionsurface. The hollow glass or other composition microspheres, can vary inparticle size from a 100 micrometer in diameter hollow glass or othercomposition microsphere 400 to a 2 micrometer-diameter hollow glass oranother composition microsphere 401. A hollow glass microsphere used asthe basis for this disclosure of 18 micrometers in diameter 405 is shownfor completion. These hollow glass or other composition microspheres inthe size range of 100 micrometers to 2 micrometers are added to thethermoelectric semiconductor powder mix at 5 to 8 weight percentconcentrations. In one successful formulation using 3M brand iM30Khollow glass bubbles, these particles are constrained within the sizerange from 2-37 micrometers in diameter with an average diameter of 18micrometers, and 30-50 million microsphere particles are incorporatedper gram of semiconductor material. These microspheres act tosignificantly reduce the cohesive nature of the semiconductor powdermaterial and therefore ease the uniform transport of the thermoelectricsemiconductor powder material through the powder feed tube and into thecold-spray nozzle.

FIG. 4B is an example of the smaller particles created in athermoelectric semiconductor powder mix adhering to the hollow glass orother composition microspheres of FIG. 4A which facilitates thesupersonic cold-spray deposition of these very small, micron tonanometer sized polycrystalline thermoelectric semiconductor materials.Because of Van der wall and other forces, the smaller, less than threeto four micrometer sized thermoelectric semiconductor particles 402 areattracted to the surface of the hollow glass or other compositionmicrospheres, varying in size from the 100 micrometer diameter hollowglass or other composition microsphere 400 to the hollow glass or othercomposition microsphere of a diameter within the particle size range 405and to the 2 micrometer diameter hollow glass or other compositionmicrosphere 401. Together they create a particle with sufficient massand drag coefficient to transit the cold-spray supersonic bow shock andimpact the surface thus aiding in the deposition of the nanometer tosmall micrometer sized thermoelectric semiconductor particles that areadhering to the hollow glass or other composition microspheres of FIG.4A onto the deposition surface and onto previously depositedsemiconductor material. The hollow microspheres also help in thetransport of irregular shaped, 3-5 micron sized, thermoelectricsemiconductor particles 403 to the deposition surface due to mechanicaland attractive forces between the thermoelectric semiconductorparticles. Cold spray testing and scanning electron microscopy of coldsprayed thermoelectric semiconductor materials performed at LawrenceLivermore National Laboratory has shown that these hollow glassmicrospheres do impact the surface, but they are not incorporated intothe deposited thermoelectric material and therefore do not change thethermoelectric properties of the deposited material.

Testing forming the basis for this disclosure show that successfulsupersonic cold-spray deposition of thermoelectric semiconductormaterials therefore requires a process where a controlled range of muchsmaller and irregular shaped particles than are used for metal coldspray applications are created. These particles are generally less than10 micrometers in maximum equivalent spherical dimension and withspecific particle size distributions down to the submicron scale,examples of which are shown in FIGS. 8A, 8B and 8C. The particles arethen accelerated to supersonic velocity in the range from 750-950 metersper second and delivered to a chemically etched or mechanicallyroughened surface. These small, highly irregular shaped particles do notplastically deform on impact, and instead the larger and smallerparticles mechanically entangle and knit together at impact to form adense, very small grain sized material. Cold spray deposition ofthermoelectric semiconductor materials depends on a small percentage ofthe larger sized, irregularly shaped, particles having sufficient energyto wedge or lock into the surface at impact without fracturing wherethey form a framework where the lower impact energy, smaller, lowmicrometer to nanometer sized particles can interlock together.Subsequent impacts of the larger and smaller particles then continue tobuild up and compact the deposited material to near theoretical density.

Supersonic cold-spray deposition of bismuth telluride and otherthermoelectric semiconductors require that the powder materials haveonly a very small percentage of the particles with a maximum equivalentspherical size greater than 10 micrometers and a controlled particlesize distribution from 0.1 micrometers to 10 micrometers. Thesemiconductor material powders need to be created using a process thatdoes not cause compositional or phase changes within the thermoelectricsemi-conductor materials. One that limits surface oxidation, limits theparticle's major dimension to below approximately ten micrometers, onethat creates a controlled particle size range that is tailored to thecold-spray system operating parameters, and one that can be applied toboth N-type and P-type semiconductor materials that utilize very smallquantities of dopants to create their beneficial thermoelectricproperties.

FIG. 5 is a flowchart of a process used to produce a thermoelectricsemiconductor powder that can be successfully deposited using thesupersonic cold-spray process. The process begins at Step 500 wherecrystalline materials not limited to but including bulk thermoelectricmaterial billets of bismuth telluride, antimony telluride orTetrahedrite type formulations are procured or directly fabricated fromthe elements using established thermal or mechanically induced crystalgrowth processes. In addition, naturally occurring, Tetrahedrite bearingrocks from the tailings of copper mines have been successfully used.Then the process proceeds to Step 501 where these materials aremechanically reduced to small chunks or nuggets below an approximately#4 standard mesh in size. Then the process proceeds to Step 502 wherethe resulting material is then high energy mechanically ball milled.Successful high energy mechanical ball milling in Step 502 includes butis not limited to ball milling the Step 501 material in 100 milliliter(ml) alumina vessels using an Across International Planetary Ball Mill.Twenty, 10 mm diameter and thirty, 6 mm diameter alumina balls are addedtogether with 50 grams of thermoelectric semiconductor fabricated perStep 501. Ten ml of denatured alcohol is then added to the aluminavessel before sealing and then planetary ball milling the mixture at 600RPM for a total of three hours and reversing the direction of the millevery thirty minutes during that three-hour period. The process proceedsto Step 503 where the resultant material is then dried at 60 degreescentigrade, removed from the vessel and separated from the milling ballsyielding a dried powder material. The process proceeds to Step 504 wherethe dried powder material is then sieved through a standard #400 meshsieve. The powder material that has passed through the sieve in step 504proceeds to Step 505 where this powder material is subjected to a secondthree hour long high-energy-ball-milling cycle that reverses directionevery thirty minutes as described in Step 502 using an AcrossInternational Planetary Ball Mill with fifty grams of the sievedmaterial added to the 100 ml alumina vessel along with 20, 10 mmdiameter and thirty, 6 mm diameter alumina balls and 10 ml of denaturedalcohol to further reduce the maximum particle size. The processproceeds to Step 506 where the further reduced maximum particle sizepowder material yielded from Step 505 is then dried at 60 degreescentigrade, and then removed from the vessel and separated from themilling balls, yielding a potential thermoelectric semiconductor powderwith a desired volumetric particle size distribution The processproceeds to Step 507 where a sample size of the potential thermoelectricsemiconductor powder is removed from the potential thermoelectricsemiconductor powder for analysis.

The process proceeds to Step 510 where the sample size of the potentialthermoelectric semiconductor powder is submitted for particle sizeanalysis.

Volumetric particle size distribution and physical particle sizeanalysis testing using a Malvern MasterSizer 2000 laser diffractionsystem with a dry dispersion technique forming the basis for thisdisclosure has demonstrated that a specific volumetric particle sizedistribution is required for the successful supersonic cold-spray ofthermoelectric semiconductor materials such as but not limited tobismuth telluride, antimony telluride and Tetrahedrite type materials.If the powder has a significant volumetric percentage of equivalentspherical particles greater than approximately 15 micrometers, thenduring the supersonic cold-spray process the larger crystallineparticles will sandblast the surface instead of adhering. Conversely, ifthe powder is composed only of particles less than 5-6 micrometers inequivalent spherical diameter, then limited to no deposition will occur.The details of this optimum particle size distribution forming the basisfor this disclosure for bismuth telluride, antimony telluride andTetrahedrite formulations is not limited to but includes the measureddistributions shown in FIGS. 8A, 8B, and 8C.The process proceeds to Step 515 where a determination is made if theparticles measured in Step 510 show that the volumetric particle sizedistribution is within the desired range. If the volumetric particlesize distribution is not within the desired range, then the processproceeds to Step 560 where a determination is made if the physicalparticle sizes are too large. If the physical particle sizes are toolarge the process proceeds to Step 520 where an additional high energymilling cycle occurs. The duration and intensity of this additional highenergy milling cycle is based on the crystalline material type andsizing data measured in Step 510. Higher energy ball milling systems, orlarger size ball milling vessels require adjustment to the total millingtime of Step 502 and Step 505 to achieve the optimum particle sizedistribution. To develop the optimum particle size distribution forother types crystalline materials requires total milling time of Step502 and Step 505 being adjusted to account for the material hardness andthe crystalline structure, and may require an additional milling cycle.Testing using bismuth telluride, antimony telluride and naturalTetrahedrite bearing rocks has shown that the double ball millingprocess as described in Step 502 through Step 506 will produce a powderthat can be successfully deposited using the supersonic cold-sprayprocess.If the determination made in Step 560 that the physical particles aretoo small, the process proceeds to Step 590 and the potentialthermoelectric semiconductor powder is stored until it can bereprocessed for potential recovery and reuse.If it is determined in Step 515 that the volumetric particle sizedistribution is within the desired range, then the potentialthermoelectric semiconductor material is identified as an acceptablethermoelectric semiconductor powder for the cold-spray process. Althoughthe acceptable thermoelectric semiconductor powder can be successfullycold sprayed as is, it is important to note that the highly cohesivenature of the acceptable thermoelectric semiconductor powder inhibitsthe transport of the powder into the supersonic cold-spray nozzle shownin FIG. 7 and can also result in clogging of the brass inlet tube 609shown in FIG. 7. To improve the powder flow characteristics, hollowglass or other composition microspheres as described in FIG. 4 can beadded to the powder prior to storage or prior to cold spraying.The process proceeds to Step 525 and a determination is made regardingwhether the acceptable thermoelectric semiconductor powder will bestored as is or if it will be stored with the addition of hollow glassor other composition microspheres.If it is determined in Step 525 that the acceptable thermoelectricsemiconductor powder should be stored without the addition of hollowglass or other composition microspheres, then the process proceeds toStep 530 where the acceptable thermoelectric semiconductor powder isstored in hermetically sealed containers for later use in the processdescribed in FIG. 6.If it is determined in Step 525 that the acceptable thermoelectricsemiconductor powder should be stored with the addition of hollow glassor other composition microspheres, then the process proceeds to Step 535where the addition of hollow glass or other composition microspheres areadded to the acceptable thermoelectric semiconductor powder. Themicrosphere addition process used for bismuth telluride, antimonytelluride and Tetrahedrite type semiconductor powders forming the basisof this disclosure used 3M brand iM30K hollow glass microspheres addedin the 5 to 8.2 weight percent range during Step 535 of this process.The process proceeds to Step 540 where the powder mix yield of Step 535is roller-mill mixed for three hours. The process proceeds to Step 545where the resultant powder mix of Step 540 is stored in hermeticallysealed containers for later use in the process described in FIG. 6. Thishollow glass microsphere additive significantly improves the ease ofpowder transport into the cold-spray nozzle, and it also improves thereliability and uniformity of the deposition by allowing a morecontrolled particle injection rate into the supersonic gas stream. Thisis especially important when using vibratory feed systems for thepowder.

FIG. 6 is a flowchart of steps in an overall supersonic cold sprayprocess and system design parameters developed for thermoelectricsemiconductor and other crystalline materials as described in thecurrent disclosure.

The process has been developed using nitrogen gas, but it is expectedthat helium, and mixtures of helium and nitrogen gas can be used, andfor specific applications the use of argon or compressed air might bedesirable. In the process forming the basis for this disclosuredeveloped for bismuth telluride, antimony telluride, and Tetrahedritetype thermoelectric semiconductors, the process begins at Step 600 wherenitrogen gas is pressurized to 0.5-0.9 MPa. The process proceeds to Step601 where the gas from Step 600 is directed into a gas heater where itis heated to 475-550 degrees centigrade. Using Bismuth telluride orantimony telluride thermoelectric semiconductor materials the optimumgas pressure is approximately 0.70 MPa, and the optimum gas temperatureentering the convergent section of the nozzle is 500 degrees centigrade.The process proceeds to Step 602 where the heated gas from Step 601 isthen fed into a de Laval type converging-diverging nozzle with a throatdiameter between 1-2 mm diameter, exit area to throat area expansionratio of between 6-10, a divergence to convergence length ratio of 3 to1 with a divergence full cone angle of 12 degrees, and powder injectiontube diameter of 1.5 mm and injection angle of 57 degrees. The nozzleassembly design producing the best results for supersonic cold spray ofbismuth telluride and antimony telluride materials occurred when usingthe gas conditions stated above with a converging-diverging nozzle withthroat diameter of 1.5 mm, a convergence-divergence length ratio of 1 to3, a total divergence full cone angle of twelve degrees, and an exitarea to throat area expansion ratio of 10. At the stated conditions, thepeak nitrogen gas velocity is approximately 850 meters/sec. This highvelocity gas flow creates a suction at the 609 1.5 mm diameter powderinlet tube injection location.Step 603 is a repository for the thermoelectric semiconductor powderfabricated in Step 530 of FIG. 5 or the thermoelectric semiconductorpowder fabricated in Step 545 of FIG. 5. The process proceeds to Step604 where the powder held in the repository of Step 603 is thermallytreated at 60 degrees centigrade for fifteen minutes. The processproceeds to Step 605 where the heated powder of Step 604 is sievedthrough an #80 mesh sieve to break up large agglomerations that may haveformed in the powder prior to proceeding to the next step. The processproceeds to Step 606 where the sieved powder of Step 605 is loaded intoa powder feed system. In Step 607, an air or other gas such as nitrogenor argon source is fed into the powder feed system of Step 606 atatmospheric pressure. The outlet of the powder feed system is connectedto a flexible three mm ID silicone rubber tube 608 which is connected toa 1.5 mm ID brass inlet tube 609 that is mounted to the nozzle assemblyof Step 602. The brass inlet tube 609 outlets into the 1.5 mm diameterpowder inlet port located toward the end of the diverging section of thenozzle. Thermoelectric semiconductor powders developed as shown in FIG.5 that are loaded into the powder feed system Step 606 still retain ahigh degree of cohesiveness and therefore maintain a high angle ofrepose, and resistance to flow. Various powder feed system types havebeen used to successfully transfer the powder into the nozzle assemblyincluding but not limited to a repetitive pulse feed technique wheresmall quantities in the order of 0.1-1.0 grams of the powder areintermittently sucked into the nozzle by repetitive opening and closingthe port to the 608, silicone rubber tube. Vibratory feed systems,fluidized bed feed systems, and pressure feed systems currently used inmany commercial cold spray systems for metal deposition are noteffective in aiding the transport of highly cohesive thermoelectricsemiconductor or other polycrystalline powders into the 608 tubing. Inanother embodiment, successful transport of the cohesive powder into thenozzle system has been demonstrated using ultrasonic vibration of thepowder in Step 606 to facilitate powder transport into the 608, siliconerubber tube by restricting the flow of the powder produced in Step 605into the Step 606 powder feed system. Powder from Step 605 fed into anultrasonic Step 606 powder feed system at feed rates of 1-4 grams persecond has yielded successful deposition of bismuth Telluride andantimony telluride powders. Mixing of the powder in Step 606 into aslurry using denatured alcohol and letting the suction created by thenozzle draw the slurry through the 608, silicone rubber tube into thenozzle is another embodiment that has been used successfully for bismuthtelluride and antimony telluride formulations.The outlet of the divergent section of the nozzle assembly of Step 602is connected to a nozzle extension tube 610 with an inside diameterwhich maintains the throat area to exit area of the converging-divergingnozzle. For the nozzle used as the basis of this disclosure, theextension tube had a 5 mm inside diameter. The extension tube allows theparticles of the semiconductor powder to be mixed into and acceleratedto supersonic velocity by the expanded gas stream, and the tube directsthe gas and powder mix toward the surface of the substrate. Analysisperformed at Lawrence Livermore National Laboratory has shown that thenanometer to micrometer sized powder material particles rapidlyaccelerate to supersonic velocity within 5-6 centimeters after theyenter the diverging section of the nozzle assembly of Step 602.Successful supersonic cold-spray depositions of bismuth and antimonytelluride formulations have been made with straight extension tubes oflengths from 5 to 20 centimeters, and with a 36 centimeters long coiledextension tube. The best results were achieved using an extension tubelength of 10.5 cm. Testing forming the basis of this disclosure has alsoshown that the shape of the deposited thermoelectric material can becontrolled by changing the shape of the extension tube exit whilemaintaining the desired exit tube area. Successful deposition of anelongated, non-circular section of P-type bismuth telluride materialoccurred when using an extension tube with an oblong shaped outlet of2.5 mm wide by 8 mm long thus maintaining the desired nozzle throat areato exit area ratio.In Step 611 the end of the extension tube is maintained a specificdistance from a substrate surface where the thermoelectric semiconductormaterial is to be deposited. Testing using the powder preparation andthe supersonic cold spray system design and operating parameters formingthe basis for this disclosure has shown that little variation in thethickness of the deposited material occurs when using standoff distancesfrom 0.65 centimeters to 1.9 centimeters. The optimum step 611 standoffdistance using bismuth telluride and antimony telluride materials is1.3-1.6 centimeter from the substrate surface. The process then proceedsto Step 612 where the supersonic gas stream with the entrained particlesis then directed toward the substrate material which can include but isnot limited to metallic materials such as aluminum, copper, stainlesssteel, and nickel; Ceramic materials such as alumina, aluminum silicate;borosilicate glass and quartz; syntactic silicone resin foams, and othersemiconductors and semi-metal materials. By varying the composition ofthe thermoelectric semiconductor powder being sprayed, layeredthermoelectric materials can be fabricated that optimize the performanceof the thermoelectric generator being fabricated. Using 3D roboticpositioning technology, this additive manufacturing process for thesupersonic cold-spray deposition of thermoelectric semiconductormaterials described in this disclosure can be combined with existingmetal supersonic cold-spray technology to fabricate completethermoelectric generator devices on complex shaped waste heat sources.

FIG. 7 is a cross section of the nozzle assembly of FIG. 6 Step 602which has been used for the successful deposition of bismuth telluride,antimony telluride and Tetrahedrite P-type and N-type thermoelectricsemiconductor and other crystalline type materials. While thissupersonic cold spray nozzle design has similarities to nozzlescurrently in use for the supersonic cold-spray deposition of metallicmaterials, there are several unique differences that contribute to theability to cold spray near theoretical density thermoelectricsemiconductor materials as claimed in this disclosure. A cross sectionof a modified machined brass de Laval type convergent-divergent nozzleassembly 700 comprising a gas feeder section 730, a convergent-divergentsection 740, a powder entry section 750, the brass tube 609, and an exitsection 780. The gas feeder section 730 comprising a first uniform area701 having a top side 731 and an opposing side 732 being 11 millimetersin internal diameter and 4.1 centimeters in length and a first sharplytapered area 702. The first sharply tapered area 702 extends from theopposing side 732 of the first uniform area 701 and then being sharplytapered at an angle of 60 degrees from a center axis of theconvergent-divergent nozzle assembly 700 to a 5 mm diameter where ittransitions into a top side of the convergent portion 703 of theconvergent-divergent section 740. The convergent-divergent section 740comprising a convergent portion 703, a throat section, 704, and adivergent section 705. The convergent portion 703 consists of a top side742, an opposing side 743. The top side 742 of the convergent portion703 is immediately tapered at a total angle of 22 degrees for 9 mm tothe opposing side 743 of the convergent portion 703. The opposing side743 transitions to a top portion 746 of the throat section 704. Thethroat section 704 being 1.5 mm in diameter and extending to 3 mm inlength to an opposing side 747 of the throat section 704 transitions toa top side 748 of the divergent section 705. The divergent section 705is machined at a total cone angle of 12 degrees from the center axis for18.3 mm in length to an opposing side 749 to achieve a throat area toexit area ratio of 10.

The powder entry section 750 comprising a powder entry hole 706 and thebrass inlet tube 609 as discussed in FIG. 6. The powder entry hole 706being 1.5 mm in diameter intersects the divergent section 705 of thenozzle at angle of 57 degrees 707 from the central axis and 14.3 mm fromthe opposing side 747 of the throat section 704. The brass inlet tube609 being of an internal diameter of 1.5 mm and being of an externaldiameter of 3 mm.The powder feed system of FIG. 6, S606, attaches to the de Laval typeconvergent-divergent nozzle assembly 700 by inserting the siliconerubber tube 608 onto the brass inlet tube 609. The exit section 780comprising a constant 5 mm internal diameter section being 1.4 cm inlength 710, a stainless-steel extension tube 711, and an AN4 typecompression fitting 712. The constant 5 mm internal diameter sectionbeing 1.4 cm in length 710 extends from the opposing side 749 of thedivergent section 705 to a top side 785 of the stainless-steel extensiontube 711. The stainless-steel extension tube 711 being of an ¼ inchexternal diameter and being of an internal diameter of 5 mm and being anominal length of 10.5 cm. The AN4 type compression fitting 712 attachesand detaches the stainless-steel extension tube to the modified machinedbrass de Laval type convergent-divergent nozzle assembly 700.Nitrogen gas at a nominal pressure of 0.7 MPa and temperature of 500degrees centigrade enters the modified machined brass de Laval typeconvergent-divergent nozzle assembly 700 at the top side 731 of thefirst uniform area 701 of the gas feeder section 730. The thermoelectricsemiconductor particles fabricated as described in FIG. 5 and FIG. 6 aredrawn into the powder entry section 750 from the silicone rubber tube608 into the brass inlet tube 609 as suction is created at the powderentry hole 706. The moderate pressure, high temperature gas in the gasfeeder section 730 rapidly expands and cools in the divergent section705 of the convergent-divergent section 740 of the modified machinedbrass de Laval type convergent-divergent nozzle assembly 700. This gasflow creates suction at the powder entry hole 706 which pulls the 709thermoelectric semiconductor powder particles from the brass inlet tube609 into the supersonic gas stream. The gas and rapidly acceleratingentrained particles then continue through the exit section 780, firstentering the constant 5 mm diameter section 710 and then enter thestainless-steel extension tube 711 prior to being deposited onto asubstrate material as described in FIG. 6.Aspects of this supersonic cold spray nozzle design which aid in thedeposition of near theoretical density thermoelectric semiconductormaterials with particle sizes in the 0.1-10 μm range as shown in FIG.8A, are the throat section being 1.5 mm in diameter, theconvergence/divergence length ratio of 1 to 3, a nozzle expansion arearatio of 10, the divergent section total cone angle of 12 degrees fromthe center axis, the powder entry hole being 1.5 mm in diameter,intersecting the divergent section of the nozzle at an angle of 57degrees from the central axis, and being 14.3 mm from the opposing sideof the throat section.

FIG. 8A is a graph of a volumetric particle size distribution of P-typebismuth telluride particles that have been and can be successfullycold-sprayed using the processes and equipment design parameters definedwithin this disclosure.

The particle size distribution was generated using a Malvern MasterSizer2000 laser diffraction system with a dry dispersion technique withbismuth telluride powders fabricated per the process defined in FIG. 5.Although the laser diffraction measurement technique does not fullyaccount for the non-spherical aspect of the crystalline particles, ithas been shown to be effective in defining an acceptable volumetric sizerange distribution of the particles, which can be cold-sprayed.FIG. 8B is a table of cumulative volume % less than indicated particlesize for both P-type and N-type bismuth telluride thermoelectricsemiconductor powders that can be used for successful deposition usingthe cold-spray process of this disclosure. FIG. 8B shows the cumulativevolume % less than indicated size where volumetrically 10% of both theN-type and the P-type particles have an equivalent spherical diameter ofless than 0.62 μm; that volumetrically 50% of the N-type and P-typeparticles have as measured equivalent-spherical-diameters less than 2.23μm for the N-type and 2.56 μm for the P-type; and that volumetrically90% of the bismuth telluride particles have an as measuredequivalent-spherical-diameter less than 8.33 μm for the N-type and 10.99μm for the P-type.FIG. 8C is a table of cumulative number % less than indicated size forboth P-type and N-type bismuth telluride thermoelectric semiconductorpowders that have been and can be used for successful deposition usingthe cold-spray process of this disclosure. FIG. 8C shows the cumulativenumber % less than indicated size data for the same powder materialsshown in FIG. 8B. Several assumptions must be made when using thevolumetric percent data to calculate a cumulative number percent usingthis laser diffraction type particle size measurement technique, thenumber percent data is, therefore, simply representative of the factthat most of the particles in the mix made using the planetary ballmilling process defined in FIG. 5 are significantly less than 1.0 μm inequivalent-spherical-dimension.

FIGS. 9A, 9B, 9C and 9D shows a set of photographs and tables displayingthe effects of different volumetric particle size distributions of thethermoelectric semiconductor powders on the ability to successfullydeposit P-type bismuth telluride thermoelectric semiconductor materialsusing the supersonic cold-spray process as disclosed.

FIG. 9A is an image of P-type bismuth telluride supersonic cold-spraydeposition 903 using powder fabricated per FIG. 5 through Step 530. Thispowder had the measured volumetric particle size distributions of thoselisted in FIG. 9C.

FIG. 9B is an image of P-type bismuth telluride cold spray deposition905 using powder processed per FIG. 5 through Step 504 only andeliminating the remaining steps of the process. Using only the singlehigh energy ball milling cycle of Step 504 resulted in the P-Type powderhaving generally larger particles with the measured volumetric particlesize distributions of those listed in FIG. 9D, and those largerparticles interfere with the successful deposition process; thus showingthat using a mechanical milling process with sufficient milling time andmilling intensity is critical to creating a powder with a specificmaximum particle size and a volumetric size distribution that isnecessary for a successful cold-spraying process.FIG. 9C is a table of particle size data summary cumulative volume %less than indicated size for P-type bismuth telluride thermoelectricsemiconductor powders that were used for successful deposition shown inFIG. 9A using the cold-spray process of this disclosure.FIG. 9C shows the cumulative volume % less than indicated size wherevolumetrically 10% of the P-type particles have an equivalent sphericaldiameter of less than 0.62 μm, that volumetrically 50% of the P-typeparticles have equivalent spherical diameters less than 2.56 μm, andthat volumetrically 90% of the P-type particles have equivalentspherical diameters less than 10.99 μm.FIG. 9D is a table of particle size data summary cumulative volume %less than indicated size for P-type bismuth telluride thermoelectricsemiconductor powders that were used for the surface cratered,unsuccessful cold-spray deposition shown in FIG. 9B.FIG. 9D shows the cumulative volume % less than indicated size wherevolumetrically 10% of the P-type particles have an equivalent sphericaldiameter of less than 2.5 μm, that volumetrically 50% of the P-typeparticles have equivalent spherical diameters less than 13.3 μm, andthat volumetrically 90% of the P-type particles have a sphericaldiameter less than 26.7 μm. The powder material for FIG. 9C used in thecold spray deposition shown in FIG. 9A, and the powder material for FIG.9D used in the low quality cold spray deposition shown in FIG. 9B weremade from material removed from the same billet of P-type bismuthtelluride polycrystalline material by using the Across Internationalplanetary ball milling equipment and the generalized process steps asdefined in FIG. 5. The powder material of FIG. 9C was subjected to thecomplete process as described in FIG. 5, through Step 530. The powdermaterial of FIG. 9D was fabricated using the portions of the processdescribed in FIG. 5 through Step 504, thus generating a semiconductorpowder with larger particles that are still in the lower end of the sizerange normally used for cold-spray of metallic materials. Bothdepositions were made using the cold-spray nozzle design shown in FIG. 7with nitrogen gas at an input temperature of 500 degrees C., and aninput gas pressure of 0.7 MPa. All other test parameters such as powderfeed method, nozzle standoff distance and substrate were identical. Theresults of FIG. 9A were achieved using a bismuth telluride powder withthe cumulative volume distribution of FIG. 9C.FIG. 9A shows a uniform, near theoretical density deposition 903 of thesprayed, P-type thermoelectric material was obtained using a bismuthtelluride powder with the cumulative volume distribution of FIG. 9C.FIG. 9B, shows a porous, cratered and disrupted deposition 905 whichdemonstrates how the use of a powder with the cumulative volumedistribution of FIG. 9D, with generally larger particles disrupts thecold spray deposition process. FIG. 9B demonstrates that any initialdeposition that occurs is quickly cratered by the impact of particlesabove 15 μm in equivalent spherical diameter. These larger particleshave sufficient energy on impact to effectively crater and then“sandblast” away any already deposited thermoelectric semiconductormaterial. Successful cold spraying, FIG. 9A of thermoelectricsemiconductor and other crystalline materials requires particle sizesmuch smaller than the particle size distribution range normally used formetallic materials, which is one of the findings forming the basis ofthis disclosure.

FIGS. 10A, 10B, and 10C show comparison graphs of thermoelectricproperty data generated at Lawrence Livermore National Laboratory onbismuth telluride samples taken from polycrystalline billets andsupersonic cold-sprayed bismuth telluride materials which used the samepolycrystalline billets to produce powders fabricated per the process ofFIG. 5 of this disclosure.

FIG. 10A shows a comparison of the Seebeck coefficient over the rangefrom 0-400 degrees Kelvin for cold sprayed bismuth telluride P-typematerial 1001 compared to P-type bismuth telluride material 1002 from anas manufactured polycrystalline billet. This data verifies that the coldsprayed P-type material retains the strong positive value Seebeckresponse of the precursor billet material.FIG. 10B shows the comparison of the Seebeck coefficient versustemperature over the range from 0-400 degrees Kelvin between for coldsprayed bismuth telluride N-Type formulation 1004 compared to N-typebismuth telluride material 1003 from an as manufactured polycrystallinebillet. The measured negative Seebeck response for the cold-sprayedN-type formulation 1004 is reduced compared to the values measured forthe precursor billet material 1003 but still show a strong negativevalue Seebeck response.This data verifies that cold-sprayed N-type formulation 1004 materialwhen manufactured per the processes defined in this disclosure retain ornearly retain the high μV/K response of the precursor billet material.FIG. 10C is a graph of the measured thermal conductivity comparisonbetween P-type bismuth telluride billet material 1005 and P-typesupersonic cold-sprayed material 1006 showing that the cold-sprayedmaterial has a beneficial lower thermal conductivity over the range from50-400 degrees Kelvin. This lower thermal conductivity is beneficial inTEG applications where the peak hot side temperature is less than 400degrees Kelvin.

FIG. 11A, FIG. 11B, FIG. 11C, FIG. 11D, FIG. 11E, and FIG. 11F showexamples of various metals, glass, quartz, ceramic and silicone foambased materials that can be successfully used as the substrate materialfor the cold spray deposition of thermoelectric semiconductor materials

FIG. 11A shows a chemically etched glass slide 1100 on which asupersonic cold-sprayed line 1101 of P-type bismuth telluride has beendeposited using the processes and equipment described in thisdisclosure. The cold-spray process generates a light overspray coating1102 on the substrate surface which can be eliminated by masking of thesubstrate surface prior to spraying or by removing the overspray laterby machining. This process works equally well on a chemically etchedglass or quartz substrate surfaces.

FIG. 11B shows P-type bismuth telluride material cold spray deposit 1104on a small section of a 6061T3 aluminum substrate 1103.

FIG. 11C shows P-type bismuth telluride material cold-spray deposit 1106on a small section of a 110-ETP alloy copper plate substrate 1105.

FIG. 11D shows P-type bismuth telluride material cold spray deposit 1108on a small section of a 304 alloy, stainless steel substrate 1107.

FIG. 11E shows P-type bismuth telluride material cold-spray deposit 1110on a low-density, 0.60 grams per cubic centimeter, syntactic hightemperature capability silicone resin foam 1109. FIG. 11F shows P-typebismuth telluride material cold spray deposit 1116 on a moderatedensity, 0.80 grams per cubic centimeter, aluminum silicate ceramicsubstrate 1115. A small amount of accumulated overspray material 1117can be eliminated by prior masking of the surface or post cold spraydeposition machining of the deposited thermoelectric semiconductormaterial.

FIG. 12 is an image of supersonic cold-spray deposition of both N-typeand P-type bismuth telluride thermoelectric semiconductors on a circularcross-section tube demonstrating the ability to cold-spray depositthermoelectric semiconductors on complex shaped surfaces as described inthis disclosure.

One of the specific benefits derived from an ability to supersonic coldspray thermoelectric semiconductor materials is that the methods andprocesses described in this disclosure enable the additive manufacturingof complete thermoelectric generators (TEGs) directly onto complexshaped thermal surfaces, such as pipes and other industrial processequipment. FIG. 12 shows how the processes disclosed can be used todeposit the thermoelectric elements required for a single thermoelectricthermocouple directly onto a copper pipe 1200, wherein a single N-typebismuth telluride cold-sprayed element 1201, and single P-typecold-sprayed bismuth telluride element 1203 being directly applied tothe copper pipe 1200 with the area of the copper tube between the twoelements 1202 protected by a Kapton® film. The disclosed cold-sprayprocess can deposit small spots of material, annular rings or lines ofmaterial, as well as large area depositions. Multiple N-type and P-typeelements can be deposited and then wired in series or parallel toproduce the electrical output required These disclosed processes, whencombined with existing cold spray processes and other establishedmanufacturing processes, will enable the additive manufacturing ofcomplete multi-thermocouple TEGs systems on to thermal sources of anyshape and size. TEGs with just a few thermocouples can be fabricatedwhich convert low grade heat from industrial sources to power Internetof Things (TOT) sensors, transmitters and other devices, as well aslarge area TEG systems with many thousands of thermocouples that canrecover megawatts of energy from waste heat sources in the electricitygeneration, transportation, and industrial economic sectors of the U.S.and the world's economy. These processes also directly apply toThermoelectric Cooler (TEC) systems application in all economic sectors.The methods and processes are not limited to thermoelectricsemiconductor materials but also apply to a wide range of othercrystalline functional materials.

Controlling the exact particle size range and the shape of thevolumetric particle size distribution curve is difficult forthermoelectric semiconductor and other energy harvesting crystallinematerials using a mechanical size reduction process such as high energyball milling. The ball milling process used for thermoelectricsemiconductor powders that can be successfully cold-sprayed can resultin a large volumetric and numerical percentage of the materialfabricated being in the nanometer size range. These very small particlesmay not gain sufficient energy during the cold spray process to reachthe surface and interlock with the semiconductor material that hasalready been deposited. Those particles are instead swept away by thegas stream and either lost or need to be recovered and reprocessed forfuture use. FIG. 13 addresses one method for reducing this loss andshows another embodiment of the current disclosure whereby theseparticles are not lost, but can reach the surface with sufficient energyto adhere to the thermoelectric semiconductor material 1304.

FIG. 13 shows another embodiment of the current disclosure using highperformance magnets to focus the cold sprayed thermoelectricsemiconductor particles and improve the deposition. Nanometer sized, andvery low micrometer sized thermoelectric semiconductor powders travelingat supersonic velocity can be influenced by strong magnetic fields. Acold spray nozzle with nozzle extension tube 1300 is nominally placed1.5 cm above the substrate deposition surface 1303. Semiconductorparticles 1301 that have been drawn into the gas stream exit the coldspray nozzle with nozzle extension tube 1300 traveling at supersonicvelocity. Approximately 3-5 mm above the substrate deposition surface1303 there is a bow shock 1302. As the semiconductor particles 1301 thathave been drawn into the gas stream pass through the bow shock 1302,they exit into a region 1315 in which the gas velocity is subsonic. Whensmaller, 1 μm or less, particles encounter the bow shock they can bequickly decelerated and then swept away by the gas stream withoutadhering to the substrate deposition surface 1303 or to previouslydeposited material 1304. Placement of a high performance, neodymium typemagnet 1305 with field strength in the range from 0.2-2 Tesla directlybeneath an opposing side of the substrate deposition surface 1310improves the thermoelectric semiconductor particle deposition on thesubstrate surface 1303 and to previously deposited material 1304 byfocusing and attracting the semiconductor particles 1301 in the gasstream. The magnetic field aids in the smaller powder material particlereaching the surface with sufficient energy to adhere to the substratedeposition surface 1303 and/or the previously deposited 1304 material.

Either discrete magnetics (e.g., ceramic or rare earth magnets) or anenergized field coil may be used, being placed beneath and opposing sideof the substrate deposition surface 1310 and the central axis of theNorth and South poles of the magnet oriented co-linear with the axis ofthe cold-spray nozzle with nozzle extension tube 1300. Testing has shownthat placing the North pole of the magnet oriented facing the opposingside of the substrate deposition surface 1310 when cold-spraying aP-type semiconductor, and the South pole of the magnet oriented facingthe opposing side of the substrate deposition surface 1310 whencold-spraying an N-type semiconductor provides the greatest enhancement.Changes in the deposition efficiency of up to 20 percent have occurredby reversing the North/South Pole orientation of the magnetic field whenspraying either an N-type or P-type material. Improvements to thedeposition level by the addition of the magnetic field have beendemonstrated in the range of 10-20 percent with field strengths as lowas 0.1 Tesla. Higher field strengths from 0.2-2 Tesla further improvethe deposition efficiency. Other magnetic field line orientations (fixedin a static orientation, or a controllably altered orientation) may alsobe used, for example to improve deposition-adhesion efficiency withrespect to coverage of 3-dimensional structures that may have featuresthat are “shadowed” with respect to a main spray direction.

A unique set of processes and equipment design considerations have beendeveloped to enable the use of the supersonic cold-spray process in thedeposition of non-malleable, brittle crystalline materials such asthermoelectric semiconductors. By judicious selection of the materialparticle size, the particle size distribution, and the cold spray systemdesign and operating parameters, nanometer to very low micrometer sizeparticles made from non-malleable, brittle crystalline materials canimpact and wedge into a roughened surface and then mechanicallyinterlock together to build up a polycrystalline material of neartheoretical density without change to the material composition andstructure.

The thermoelectric semi-conductor cold-spray processes described hereincan be used in numerous energy harvesting systems for both terrestrialand space applications. The developed powder preparation and sizingprocess, when combined with specific cold spray nozzle design parametersand the specific set of cold spray equipment operational parametersallow the significant expansion of the use of TEG systems to produceelectrical power from waste heat generated by transportation systems,industrial processes, and the energy producing sectors. The ability tocold-spray thin layers of thermoelectric semiconductor materials canmaximize the power generated per unit area for certain applications, andthe ability to cold-spray deposit these materials onto complex surfacesin any size and shaped configurations opens a much wider range of energyrecovery applications for TEG systems as well as for energy sources forInternet of Things (IOT) sensors and transmitters.

In addition to thermoelectric generator applications, the processesdeveloped are directly applicable for use in thermopile systems and forthermoelectric cooler applications.

The specific attributes of the cold spray process defined herein enablesthe additional potential for layering and incorporation of smallquantities of specific figure of merit (ZT) enhancing materials withinthe powders to significantly increase the figure of merit of the sprayedthermoelectric material.

These processes developed for thermoelectric semiconductors such asbismuth telluride, antimony telluride and Tetrahedrite type materialsalso apply to the cold-spray deposition of a variety of other energyharvesting materials such as piezoelectric materials, other sensor typecrystalline semi-conductor materials, and other polycrystallinefunctional materials.

Since the process can be used for a wide range of crystallinethermoelectric semiconductor materials it can also be used to depositcomposite N-type and P-type elements where the thermoelectric materialcomposition is changed throughout the deposition layer to enable theextraction of the maximum electrical energy from the thermal source bymaintaining a high figure of merit over the full temperature range ofthe material from the hot side to the cold side of the thermoelectricgenerating device.

The invention claimed is:
 1. A method of cold spray deposition ofparticles of brittle material onto a surface of a substrate, the methodcomprising: feeding heated gas into a nozzle, the nozzle being aconverging-diverging nozzle; adding to the nozzle a functional materialpowder comprising particles of the brittle material with a majority ofthe particles of the brittle material having particle diameters in aninclusive range of 0.1 μm to 15 μm; mixing the functional materialpowder with the heated gas in the nozzle, the particles of the brittlematerial having a functional property; emitting a mixture of the heatedgas and the functional material powder from the nozzle at a supersonicvelocity toward the substrate; and coating at least a portion of thesurface of the substrate that receives the particles of the brittlematerial emitted from the nozzle while maintaining the functionalproperty of the particles of the brittle material, wherein the coatingstep includes adhering the particles of the brittle material to oneanother.
 2. The method of claim 1, wherein the mixing includes addinghollow microsphere particles to the mixture.
 3. The method of claim 2,wherein the hollow microsphere particles include glass particles.
 4. Themethod of claim 1, wherein the functional material powder includes athermoelectric semiconductor material.
 5. The method of claim 4, whereinthe thermoelectric semiconductor material includes at least one ofbismuth telluride, antimony telluride, or Tetrahedrite type coppersulfosalts.
 6. The method of claim 1 wherein the functional materialincludes at least one of a N-type semiconductor material and a P-typesemiconductor material.
 7. The method of claim 1, wherein the functionalmaterial powder includes a piezoelectric material.
 8. The method ofclaim 1, wherein the gas being nitrogen, helium or a gaseous mixturethat includes at least one of nitrogen and helium.
 9. The method ofclaim 1, further comprising pressurizing the gas to a pressure in aninclusive range of 0.4 MPa through 3.5 MPa.
 10. The method of claim 4,further comprising repeating the adding, mixing, emitting, and coatingsteps with another powder to form a different layer of materialdeposited on top of a first thermoelectric semiconductor layer ofmaterial deposited on the surface of the substrate.
 11. The method ofclaim 10, wherein the first thermoelectric semiconductor layer and thedifferent layer both comprising N-type semiconductor materials, or bothcomprising P-type semiconductor materials.
 12. The method of claim 1,further comprising applying a magnetic field to the mixture while themixture moves from the nozzle to the substrate.
 13. The method of claim12, wherein the applying includes applying the magnetic field with amagnet disposed on an opposite side of the substrate from the nozzle.14. The method of claim 1, further comprising heating a gas to becomethe heated gas at a temperature in an inclusive range of 300 degreescentigrade through 650 degrees centigrade.
 15. The method of claim 1,wherein the nozzle has a convergence to divergence length ratio in aninclusive range of 1 through
 3. 16. The method of claim 1, wherein thenozzle has a nozzle expansion area ratio in an inclusive range of 7 to13.
 17. The method of claim 16, wherein the nozzle has the nozzleexpansion ratio of about
 10. 18. The method of claim 1 wherein adiameter of a throat of the nozzle is in an inclusive range from 1.0 mmto 1.5 mm and a diameter of a powder entrance tube of the nozzle is 1.5mm.
 19. A method of making a functional material powder for use in acold spray process, the method of making comprising: mechanicallyreducing a diameter size of particles in the functional material powderto be in an inclusive range of 0.1 μm to 40 μm, with a majority of theparticles being in an inclusive range of 0.1 μm to 15 μm, the particlesbeing brittle and having a functional property; applying the particlesto a ball mill with alcohol; drying the particles; storing driedparticles having a diameter in the inclusive range of 0.1 μm to 40 μm,with a majority of the particles being in the inclusive range of 0.1 μmto 15 μm; and coating a surface of a substrate by emitting the particlesfrom a nozzle at a supersonic velocity and adhering the particles to oneanother.
 20. A cold-spray deposition nozzle comprising: a gas feed thatreceives a heated gas into the cold-spray deposition nozzle; aconvergent-divergent nozzle comprising a convergent portion that is openat one end thereof to receive the heated gas from the gas feed and anarrower opposite end, a throat that receives the heated gas from thenarrower opposite end of the convergent portion, a divergent portionthat has a first end which receives the heated gas from the throat, thedivergent portion tapering from the first end to a larger second endthrough which a mixture of the heated gas and functional material powderof particle diameter sizes in an inclusive range of 0.1 μm to 40 μm areemitted, and an input port that communicates with an interior of thedivergent portion and through which the functional material powder isinput so as to mix with the heated gas in the divergent portion prior tobeing emitted from the cold-spray deposition nozzle through the largersecond end of the divergent portion, wherein the nozzle has a nozzleexpansion area ratio in an inclusive range of 7 to 13, a diameter of thethroat is in an inclusive range from 1.0 to 1.5 mm, and a diameter ofthe powder entrance tube is 1.5 mm, and the convergent-divergent nozzleis configured to emit particles of the functional material powder at asupersonic speed so as to form a coating of the particles that adhere toone another on a portion of a surface of a substrate, the particlesbeing brittle.